1. Field of the Invention
The invention relates to an adaptation layer for an ultrasound applicator which is provided for matching of the acoustic impedance of an ultrasound source to the acoustic impedance of the body of a patient.
2. Description of the Prior Art
Ultrasound applicators of this kind are widely used in medical technology to obtain information about the inner structures of tissues and organs in a patient. One problem is how to introduce the ultrasonic waves into the patient.
The piezoelectric transducer(s) of medical ultrasound applicators often comprise a material having a relatively high acoustic impedance. Materials such as ceramics of lead-zirconate-titanate have e.g., an acoustic impedance of about 30.times.10.sup.6 kg/m.sup.2 sec. On the other hand, the skin and tissue of the patient only have an acoustic impedance of about 1.5.times.10.sup.6 kg/m.sup.2 sec. To avoid to a large extent undesirable reflection at the interface between the piezoelectric transducer and the human tissue, an adaptation (or impedance-matching) layer is disposed between the transducer and the tissue.
The acoustic impedance of this adaptation layer depends on the impedance of the piezo-ceramic used as the transducer element. Therefore, the impedance should be freely selectable or adjustable within certain limits and include a range that extends from about 6 to 12.times.10.sup.6 kg/m.sup.2 sec. With natural occurring materials such an acoustic impedance is difficult to attain. For example, gases and liquids are in the range from 0 to 4.times.10.sup.6 kg/m.sup.2 sec. There are but a few substances with values above the last-named value, that is, there are practically no materials with the favorable matching impedance of about 8.times.10.sup.6 kg/m.sup.2 sec. The acoustic values of minerals, metals, etc. range between 14 and about 100.times.10.sup.6 kg/m.sup.2 sec. The range desired herein of about 8.times.10.sup.6 kg/m.sup.2 sec can only be reached with great difficulty by means of glass compounds.